Characteristics of Animals

Characteristics of Animals

• Animals are multicellular, heterotrophic eukaryotes lacking cell walls
• All animals have extracellular proteins, including collagen
• Animals have nervous and muscle tissue for movement and response
• Animal reproduction happens through sexual reproduction, and the diploid stage is dominant
• A zygote undergoes cleavage, forming a blastula
• The blastula undergoes gastrulation, forming embryonic tissue layers
• Gastrulation can result in 2 or 3 tissue layers, ultimately leading to formation of a gastrula

Embryonic Development

• Hox genes are unique to animals
• Hox genes regulate the development of body form
• The presence of Hox genes contributes to wide diversity in animal morphology

Common Ancestors

• Animal ancestors may have lived 1.2 billion–800 million years ago
• These ancestors likely resembled modern choanoflagellates
• The common ancestor was probably a colonial protist

Paleozoic Era

• The Cambrian explosion occurred 542-525 MYA
• There was a rapid increase in the diversity of animal groups
• Several hypotheses for the sudden rise in diversity include:Snowball Earth events, increase in atmospheric oxygen, predator-prey interactions, evolution of Hox genes

Animal Body Plans: Symmetry

• Asymmetry is present in sponges (Porifera)
• Radial symmetry is seen in Cnidaria
• Bilateral symmetry is present in vertebrates and arthropods
• Bilaterally symmetrical animals have dorsal (top) and ventral (bottom) sides
• They also have right and left sides, plus anterior (head) and posterior (tail) ends
• Cephalization involves the development of a head

Animal Body Plans: Tissues

• Tissues are collections of specialized cells isolated from others by membranes
• Three tissue layers include the:
  • Ectoderm which forms the outside lining, nervous tissue, and glands
  • Endoderm which forms the gut lining and digestive organs
  • Mesoderm which forms muscles and middle bits
• Diploblastic animals have two germ layers and are radially symmetrical
• Triploblastic animals have three germ layers and are bilaterally symmetrical

Animal Body Plans: Body Cavity

• Triploblastic animals can have a body cavity
• A coelom is a body cavity present in some triploblastic animals
• Body cavities may be present or absent in animals
• Coelomate animals have a body cavity formed from the mesoderm
• Pseudocoelomate animals have a body cavity formed from mesoderm and endoderm
• Acoelomate animals have no body cavity

Animal Body Plans: Protostomes vs. Deuterostomes

• Protostomes and Deuterostomes differ in the pattern of cleavage
• Protostomes exhibit spiral and determinate cleavage
• Deuterostomes exhibit radial and indeterminate cleavage
• In general, protostome development begins with spiral, determinate cleavage, while deuterostome development is characterized by radial, indeterminate cleavage
• In protostomes, the coelom forms from splits in the mesoderm (schizocoelous development)
• In deuterostomes, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development)
• In protostomes, the blastopore becomes the mouth
• In deuterostomes, the blastopore becomes the anus

Kingdom Animalia Classification

• There are about 35 animal phyla
• One hypothesis that Bilateria splits into deuterostomes and protostomes
• The split of bilateria into deuterostomia and protostomia classifications is based on morphological and developmental comparisons
• Another hypothesis based mainly on molecular data
• Another hypothesis separates bilateria into deuterostomia and 2 groups of protostomes that include ecdysozoa and lophotrochozoa

Protostomes: Ecdysozoans and Lophotrochozoans

• Ecdysozoans shed their exoskeletons in a process called ecdysis
• Lophotrochozoans have a lophophore
• Lophotrochozoans go through a distinct larval stage called a trochophore larva